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Original articles

Scabies incognito

diagnostic value of dermoscopy-guided microscopic examination

Diab, Heba M.

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Journal of the Egyptian Women's Dermatologic Society: January 2017 - Volume 14 - Issue 1 - p 56-60
doi: 10.1097/01.EWX.0000490006.68953.19
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Scabies is a contagious cutaneous infestation provoked by Sacrcoptes scabiei mite 1,2. Its diagnosis is usually easy following appropriate history taking and determination of clinical distribution of specific skin lesions. However, in patients on systemic corticosteroid treatment and with immune-compromised disorders, scabies can show atypical manifestation 3, rendering clinical findings unreliable for the diagnosis of scabies incognito. Confirmation of diagnosis is important before treatment of all contacts, many of whom may be asymptomatic 4.

Dermoscopy, originally introduced to diagnose pigmented lesions and melanoma, was additionally used as a noninvasive accessible device for the diagnosis of many skin diseases, including scabies in vivo5. The advantage of dual availability of magnification and light renders the skin top layer translucent, thus enhancing visualization of skin layers down to superficial papillary dermis and increasing the reliability of clinical inspection 4. In scabies, dermoscopy has the advantage of visualizing mites that appear as a characteristic triangular or a jet with a contrail representing the head and front pairs of legs of the S. scabiei mite 2,6. Few case reports and studies described the experiences of dermoscopy-assisted diagnosis of crusted, nodular, and classic scabies 7–9, but hardly any specific study about diagnosing scabies incognito was addressed. Moreover, there have been very few clinical studies concerning the diagnostic sensitivity of dermoscopy by comparing skin scraping ‘with dermoscopy’ with skin scraping ‘without it’ 10.

To address this limitation, this study was designed to assess the diagnostic value of dermoscopy-guided scraping and microscopy for diagnosing scabies incognito.

Patients and methods

This prospective, evaluator-blinded study included 100 patients who presented to the Dermatology Outpatient Clinic, Ain Shams University Hospitals, during the period from September 2015 to March 2016 with intractable chronic itching with different clinical manifestations ranging from visible scratch marks and excoriations to minimal or nonremarkable skin lesions. All had relapsing intensity of itching in response to empirical treatment with oral antihistamine and topical or oral steroids up to 60 mg. The study was conducted according to the Declaration of Helsinki Principles and was approved by the ethical committee of Faculty of Medicine, Ain Shams University.

Medical history included the following: the onset, course, and duration of pruritus; the presence and onset of skin lesions; the onset, course, and duration of systemic or dermatological diseases and allergies; past and recent drug intake; and family history of similar conditions.

All patients were subjected to thorough general and skin examination, including examination for possible primary skin disease, secondary scratch lesions including excoriations and ulcerations, lichenification, hyperpigmentation or hypopigmentation, and atrophic scars and ichthyotic skin changes.

Dermoscopy examination

Skin lesions were thoroughly examined using ×10 magnifications with a DermLite DL3 hand-held Dermoscope (3 Gen LLC, San Juan Capistrano, California, USA). Dermoscopic findings were photodocumented with a digital camera after being attached to the dermoscope by its specific camera adaptor.

Studied patients were then categorized into two groups: group I and group II.

Group I included patients with definite identifiable dermatological lesions confirmed by means of dermoscopy with diagnosis beyond suspicion and were excluded from the study. Their diagnoses included xerotic, atopic eczema, ordinary scabies with visible burrows and mites (Figs 1 and 2).

Figure 1
Figure 1:
Ordinary scabies with clear identification of burrow. Dermoscopy (×10).
Figure 2
Figure 2:
Mite under Dermoscopy as a brown structure (×10).

Group II included patients provisionally suspected as having scabies incognito on the basis of these criteria: personal history of chronic refractory nocturnal exacerbated itching for more than 2 weeks and not responding to empirical topical or systemic steroids and antihistamines. Positive family history of similar condition with clinical query lesions mimicking furrows, wherein mild scratching and excoriations could be seen (Fig. 3). Dermoscopy examination was not conclusive and showed hardly visible mites or brown structures (Fig.4). Patients in this group were further examined by two other physicians experienced in both dermoscopy and scraping microscopy examination in two separate rooms, wherein both patient and physicians were unaware of the results in each room.

Figure 3
Figure 3:
A 32-year old patient with chronic itching and 60 mg systemic steroids showing mild scratch marks and excoriations.
Figure 4
Figure 4:
Furrow in scabies incognito patient under dermoscopy (×10 extra ×6 of the camera zoom).

In room 1, clinical naked eye examination and scraping were carried out with timing for the whole process until diagnosis is reached as whether positive or negative for mite detection and were declared negative after 45 min. Scrapings were taken using a 13 blade scalpel from suspected lesions in predilection sites of scabies, including the buttocks, groins, trunk, under both axillae, around umbilicus, and finger webs. Scrapings were taken after putting a drop of mineral oil on the suspected sites to facilitate separation of the mite, followed by examination under microscopy with oil immersion lens to view feces of mites (scybala). This was followed by addition of 10% potassium hydroxide on scrapings for better visualization of the mite or its eggs under microscopy (Fig. 5). This was followed by referral to the second room to be examined by the second physician.

Figure 5
Figure 5:
Mite identified by means of light microscopy (×100, oil immersion lens) in skin scrapings of a burrow (after 10% potassium hydroxide).

In room 2, dermoscopy examination was carried out and dermoscopy-guided skin scrapings were obtained from suspected lesions followed by the same steps before examination under microscope with timing until diagnosis is reached as whether positive or negative.

Statistical analysis

Data were introduced and statistical analysis was performed with an IBM computer using statistical program for social science, version 17 (SPSS Inc., Illinois, Chicago, USA); quantitative variables were expressed as mean±SD, and range, whereas qualitative variables were expressed as number and percentage. The χ2-test was used to compare qualitative variables and unpaired t-test was used to compare quantitative parametric variables. McNemar’s test was used to compare paired proportions, in which sensitivity reflected the ability of the test to detect positive cases by calculating as follows: true positive/(true positive+false negative). Probability value was considered significant when P value was less than 0.05 and nonsignificant when P value was greater than 0.05.


Between July 2015 and March 2016, 100 patients (36 female and 64 male; age range 20–67 years; 38±2 years) with chronic nocturnal pruritus for 2–4 weeks (3±2.4 weeks) and positive family history of itching in 44% of them presented at the Dermatology Outpatient Clinic, Ain Shams University Hospitals. Their skin type was mainly Fitzpatrick 4 or 5. Seventy-three of 100 were included in group I with sure diagnosis and were excluded from the study. Group II included 27 patients who were suspected as having scabies incognito. As regards examination in the first room, results were obtained minimally after 25 min and were declared negative after 45 min (32±15 min). Microscopic examination of scrapings without dermoscopy was positive for the visibility of mites or its products in 12 of 27 cases (Fig. 5). Results of the second room were obtained after 5–10 min (7±2 min). Guided by dermoscopy, brown suspected structures representing mites were detected around furrows that were easier to visualize using the dermoscope lens in both typical and atypical sites such as back of ear, elbows, or knees in less than 10 min. Dermoscopy-guided scraping and microscopic examination revealed that 25 of 27 cases had a positive result detecting mites or eggs. False-negative results were obtained in room ‘1’ in 13 cases and all had skin type 5. These 13 cases were found positive when examined in room ‘2’ with dermoscopy-guided scraping. The sensitivity for dermoscopy-guided scrapings was 92.6% compared with 44.4% ‘without it’. As regards comparison of duration and time expenditure, there was a significant statistical difference on comparing time limit for examining patients with dermoscopy-guided scraping until a mite or eggs were detected with scraping without a dermascope. Thus, dermoscopy-guided scrapings increased the sensitivity and reduced the duration to reach diagnosis of scabies incognito compared with skin scraping without dermoscopy.


Ex-vivo microscopic examination of skin scrapings and detection of mites from suspected sites is the only confirmatory test for the diagnosis of scabies incognito, yet many physicians prescribe empirical antiscabetic for chronic itching, especially in areas lacking microscopes 11,12. Although diagnosis of typical scabies is considered easy by some physicians, atypical presentations may be confused with different dermatological presentations 13. These include patients under systemic steroid treatment or immune-compromised patients. Microscopic identification using conventional scraping procedures will not be practical due to the difficult visibility of skin lesions to scrap from and the relatively wide surface area to investigate by scrapping and is moreover considered invasive by some physicians, thus adding value of dermoscopy as an easy, portable, and painless method for accurate diagnosis, especially when added with microscopic examination of the guided scrapings 14.

In this prospective, evaluator-blinded study, we confirmed that the dermoscopy-guided skin scraping is superior and faster in diagnosing scabies incognito compared with scraping examinations without dermoscopy. Dermoscopy-guided skin scraping increased the sensitivity and reduced time interval until reaching diagnosis significantly. These results signify the role of dermoscopy-guided scraping and microscopic examination as a helpful tool in the early diagnosis of scabies incognito both rapidly and accurately compared with examination without dermoscopy, in which the diagnosis could be prone to error and cases might be missed causing spread of infection 11,12.

In agreement, Park et al.10 conducted a study that included 22 patients complaining of pruritus who were empirically treated with systemic or topical steroids for at least 2 weeks before they visited their clinic. They reported that 21 patients had scabies incognito and could not find mites or eggs in 12 of 21 patients without dermoscopy. They concluded that dermoscopy is a useful tool in the early detection of patients with scabies incognito. In the current study, 15 of the suspected 27 cases were found negative when examined with naked eye-guided scraping and microscopy, wherein scraping alone failed to locate the parasite or its products compared with guided dermoscopy. These 15 cases were excluded as they were true negative and were nonscabetic. Moreover, positive cases were easily scrapped using dermoscope and showed the mite, its eggs, or feces under oil immersion lens in less than 5 min. Negative results were not declared negative up to 45 min to ensure proper scraping and searching all suspected scabies sites in room 1; false-negative results were obtained in 13 of the 25 true-positive cases detected in room 2. False-negative results were more frequently obtained in patients with darker skin type 5 due to decreased visibility of the brown mite on the dark background of pigmented skin. Considering these difficulties, the detection rate of ‘with dermoscopy’ was about twice that of ‘without it’ explaining the great value of dermoscopy.

Few well-designed studies evaluated the diagnostic value of dermoscopy. Walter et al.14 and Dupuy et al.15 confirmed that dermoscopy itself is a highly sensitive method in diagnosing scabies, but the sensitivity varied in both studies. Walter et al.14 reported lower sensitivity than that reported by Dupuy et al.15 due to the darker skin type of their patients that may have affected visualization of mites. Park et al.10 in their study stated that the sensitivity of dermoscopy was higher than that in the study performed by Walter et al.14, but further studies are needed to properly evaluate whether dermoscopy use alone can have an equal accuracy and sensitivity over microscopic examination. It can unquestionably reduce the time until diagnosis to nearly half, but its precise sensitivity and accuracy, especially in darker skin types is still unresolved. As regards the technique and methods of scrapping and microscopic visualization, we used mineral oil to preserve the scybala (feces) that was the only visible diagnostic finding in some cases. For further identification of mites, potassium hydroxide was used to demonstrate eggs and mites 4 but its use instead of oil will dissolve scabies’ scybala, and so we used potassium hydroxide after mineral oil.

The true value of this small portable handheld device is the ability to magnify skin layers that in cases such as scabies incognito can decrease the time to reach diagnosis to more than 1/6.


Dermoscopy-guided scraping and microscopic examination is a sure, easily accessible, rapid approach for precise diagnosis of scabies incognito. It allows the start of prompt treatment, not only of the patient but also for all his close contacts, thus decreasing the probability of spread of infestation, as well as follow-up of treatment progress, thereby reinforcing the increasing importance of this technique in routine practice.


Conflicts of interest

There are no conflicts of interest.


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dermoscopy; microscopy; scabies incognito; scraping

© 2017 Egyptian Women's Dermatologic Society